The present invention relates to a thermosensitive recording material and more particularly to a thermosensitive recording material using a leuco dye as coloring material, which is particularly improved with respect to thermal head matching properties, color developing capability, water resistance, oil resistance and stability of the developed image.
Recently thermosensitive recording materials are employed in a variety of fields, for instance, for use with printers of computers, recorders of medical analytical instruments, facsimile apparatus, automatic ticket vending apparatus, and thermosensitive copying apparatus, since they have the following advantages over other conventional recording materials: (1) Images can be formed by simple heat application, without complicated steps for development; (2) the thermosensitive recording materials can be produced by a simple apparatus and the storage of the thermosensitive recording materials is simple and does not involve excessive costs; (3) as the support material of the thermosensitive recording materials, paper is usually used, which is rather inexpensive in comparison with other support materials, such as synthetic resin films, and when paper is used as the support material, the thermosensitive recording material has a pleasing plain-paper-like touch.
A conventional thermosensitive recording material comprises a support material such as a sheet of paper and a thermosensitive coloring layer formed on the support material, on which thermosensitive coloring layer colored images can be formed by application of heat thereto. For heat application for image formation, a thermal head is in general use. In such a conventional thermosensitive recording material, there are usually employed in the thermosensitive coloring layer a colorless or light-colored leuco dye containing a lactone ring, a lactam ring or a spiropyran ring, and a color developer which induces color formation in the leuco dye upon application of heat by the reaction with the leuco dye, since it is capable of yielding clear images with minimized fogging.
Images that can be obtained by a thermal reaction of a leuco dye serving as coloring material and a phenolic or other organic acidic material serving as a color developer capable of inducing color formation in the leuco dye have the shortcoming that they are apt to discolor when coming into contact with a plasticizer contained in wrapping films, plastic materials and rubber eraser, a cooking oil, water, organic solvents and alcohol.
In order to eliminate this shortcoming, there is proposed a method of overcoating the thermosensitive coloring layer with a water-soluble binder agent. However, the thermosensitive coloring sensitivity of the recording material is significantly reduced by such overcoating. Furthermore, such overcoating can reduce the water resistance, image stability and thermal head matching properties of the recording material.